Electronic devices commonly include printed circuit boards on which electrical components, such as integrated circuit (IC's) and other types of electrical components, are mounted and connected in particular ways to provide desired functionalities. A common approach to mount an electrical component to a printed circuit board is to use solder. More particularly, a number of solder bumps may be applied to the printed circuit board and heated, such that an electrical component can then be disposed to the solder bumps to affix the component to the circuit board.
Unfortunately, this approach to mounting electrical components to printed circuit boards can be problematic. Stresses can result from the differences in the coefficient of thermal expansion (CTE) of an electrical component and the CTE of solder, as well as from differences in the CTE of a printed circuit board and the CTE of the solder. During use of such an electrical device, for instance, if these stresses become too high, the solder may crack, causing the electrical component to no longer be properly affixed to the printed circuit board.
A solution to alleviate this problem is to include an underfill film between the electrical component and the printed circuit board of an electronic device. The underfill film itself relaxes the stresses resulting from CTE differences of the solder and the electrical component and the printed circuit board. Such stresses are thus absorbed by the film, instead of by the solder, reducing the likelihood that the electrical component may break way from the printed circuit board.
Another issue within electronic device design is the dissipation of heat. Modern IC's, for instance, can generate significant amounts of heat, which if not properly dissipated can cause failure of their electronic devices. Furthermore, electronic devices have become increasingly smaller, leading to printed circuit boards that are closely packed with electrical components. This means that using heat sinks for heat dissipation, as is conventional, can become problematic, because they may not be able to be located near the electrical components that require heat dissipation.
One solution is to add solder bumps, or balls, to the underside of a printed circuit board, which serve to dissipate heat through the printed circuit board. This approach is not overly effective, however, because the printed circuit board itself is usually not a good thermal conductive, such that the printed circuit board itself becomes a thermal insulator. Therefore, another approach, either alone or in combination with solder bumps on the underside of a printed circuit board, is to use the printed circuit board itself as a type of heat sink to dissipate heat.
For example, printed circuit boards may be manufactured using a resin that has a high thermal conductivity. Printed circuit boards using such resins are available, for instance, from Thermagon, Inc., of Cleveland, Ohio. As another example, printed circuit boards may incorporate graphite sheets to improve their thermal conductivity. Such printed circuit boards are available, for instance, from U-AI Electronics Corp., of Aichi, Japan. The graphite sheet may be caused to contact the metal or other chassis of the electronic device in question, to further improve heat dissipation, as described in the published Japanese patent application no. JP 1999-233904A, published on Aug. 27, 1999, and entitled “Heat release structure print substrate.”
However, in order for a printed circuit board to effectively dissipate heat, there must be a thermally conductive path between the electrical components and the printed circuit board in the first place. Unfortunately, the inclusion of underfill films between the components and a printed circuit board effectively results in the components being thermally insulated from the printed circuit board. That is, most underfill films are made of a resin or another material that has low thermal conductivity. Therefore, heat does not efficiently travel from the electrical components to the printed circuit board.
A limited solution is to mix materials with high thermal conductivity into the underfill film material itself to improve thermal conductivity. For example, underfill available from AI Technology, Inc., of Princeton Junction, N.J., includes aluminum particles mixed into resin to improve the thermal conductivity of the resulting underfill film. However, this solution only transfers heat to the printed circuit board itself. Even a printed circuit board with a high thermal conductivity still has a thermal conductivity lower than most heat sinks, for instance, and therefore additional heat dissipation may be required. For instance, the printed circuit boards described above using thermally conductive resins still have relatively low thermal conductivity as compared to heat sinks.
For these and other reasons, therefore, there is a need for the present invention.